This invention relates to a controller for a pressure medium especially adapted for hydrostatic steering systems of motor vehicles.
Pressure controllers for hydrostatic steering systems having relatively displaceable valve elements are well known. Such valve controllers are displaceable from a neutral position to two operating positions. In the neutral position, the valve elements establish a short circuit connection between a pressure inlet and outlet through a conduit system formed by a plurality of radial passages. One of the valve elements is drivingly coupled to the hand steering wheel while the other valve element is drivingly coupled to a fluid metering pump also operated by the hand steering wheel, through which metered pressure fluid is delivered to a steering servomotor. Such an arrangement of relatively displaceable valve elements with radial valve passages to regulate the operating pressure delivered to the servomotor is disclosed in German Patent No. AS 2156842. The radial valve passages in such a valve controller are provided with three different flow area diameters wherein pairs of passages in one valve element cooperate with the passages of the other valve element in order to effect regulation of the operating pressure during displacement of the valve elements without fluctuations in the control pressure. It is well known that such pressure fluctuations may be avoided by controlling the increase of the operating pressure medium in accordance with a predetermined pressure characteristic represented by a compound curve such as a catenary. In order to obtain such a pressure variation characteristic, the flow area of the short circuit path established in the neutral position of the valve elements, must be initially decreased at a rapid rate and subsequently at a lower rate. In order to achieve the foregoing objective, pressure controllers heretofore utilized were provided with many radial bores in both valve elements having at least three different flow area diameters. As a result of the number of radial bores required and the variation in diameters, certain constructional limitations were imposed on the design of the valve controller and certain manufacturing difficulties were introduced to provide radial bores of several sizes. The economic disadvantage associated with the aforementioned valve controllers is particularly aggravated when producing valve elements that are relatively displaceable solely in the axial or an angular direction, because of the relatively short valve adjusting path available in such valve controllers.
It is therefore an important object of the present invention to provide a pressure controller of the aforementioned type having the aforementioned advantages associated therewith but capable of being made in a more economical fashion.